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So I finally localized the horrible smell of the interior of my car, and it turns out it is the plastic parts! Here I thought it was 75 years worth of mouse droppings and mold in the headliner but as soon as I replaced one of the window crank knobs, there was the smell. Sort of a mixture between vomit and poo. Gross.

Now I probably wouldn't have started ripping out the interior if I had known it was coming from the old plastic. The issue is, I am only replacing the broken window crank knobs and door escutcheons. The whole instrument panel and radio grill is perfect (not to mention the replacement for these is something like $1500).

Is there anyway to neutralize the smell of this plastic? Any magic oil or wax I can treat it with??

You're smelling the degradation products of the Tenite. I'm surprised it's that bad, but sealing both sides in clear paint should help.

If it's a '40's era car, I doubt the smell is the plastic unless you're particularly sensitive to some compounds, which is quite possible as you can detect very low levels once sensitized.
Since the materials that are used to make the plastic are encased in the polymer matrix, they can't easily come out, but as the surface degrades and years of baking in the sun, they can degrade and come out. For years the auto industry fought the inside "window haze" and it was traced to the more volatile plasticizers in vinyl trim. The normal daily heating and cooling causes it.
If it is the plastic and has a vomit or poo smell, it could be some sulfur analog or amine break down product.
It could also be bacterial components causing it.
1. If you can't replace the knobs with NOS, clean them up and make a mold or have made and recast new with modern materials to match.
2. I'd thoroughly wash/scrub the affected parts with detergent and water and if that didn't do it, buff off the parts to remove the oxidized layer. If it smells worse during that process, you know it's the source.
I wouldn't rule out other sources though. Probably somewhere you can't easily see. This is just one of many untold unpleasant issues you have to deal with vintage cars as it's not as easy as seen on TV. I spent most of a month, disinfecting and cleaning up, just to get to a state to work on it safely. Wear a fitted activated carbon acid gas respirator, if you're really sensitive to the smell.

The photo oxidation of the compound at the part surface, whether an additive or the primary polymer, is nothing like the volatilization of the plasticizers in flexible vinyl trim later. I agree that unless you are really sensitive, it's odd that there is enough mass to be the main smell. The parts are reproduced by Yesterday's Radio.

I know the smell that is being reported very well. If you have an old Magnavox record player from the 50's the plastic 45 adapter will make you gag. I have an RCA unit and don't recall that it stinks like the Magnavox.

But that smell is indeed awful for anybody with intact senses. However, some of the latest Detroit cars stink to high heaven as well. One in particular smells like the Sharkey Landfill in New Jersey.

I have an old Craftsman screwdriver and it stinks the same way. I don't smell it in the toolbox, but if you pick it up and sniff it, it smells like bilge water.

It's butyric acid and yes it reeks

Wow! I had no idea there were so many chemists on this forum! It does indeed reek and it's amazing that such a powerful, concentrated smell can come from one tiny knob.

Good news is I have bought all new knobs and escutcheons from Yesterday's Radio and haven't checked the dash/radio grill yet. With a little luck that will be ok and swapping knobs and escutcheons will do the trick. If not, I will try the detergent method recommended here then perhaps a coating of carnauba wax could seal it in?

Would it make sense to apply some kind of shellac over the original plastic to seal the smell in or prevent more off-gassing of this butyric acid?

I'd use a different coating than shellac - shellac is sensitive to moisture (think of rings on furniture tops)